U.S. patent application number 10/479880 was filed with the patent office on 2004-09-02 for fuse component.
Invention is credited to Baus, Andreas, Jollenbeck, Andre, Roder, Uwe.
Application Number | 20040169578 10/479880 |
Document ID | / |
Family ID | 7687801 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040169578 |
Kind Code |
A1 |
Jollenbeck, Andre ; et
al. |
September 2, 2004 |
Fuse component
Abstract
A fuse component includes an electrically insulating substrate
having a top surface, a thick film fuse element applied to the top
surface and a cover layer. The cover layer is made of an
electrically insulating material having good caloric conductivity.
The cover layer can be directly applied to the thick film fuse
element and the adjoining zones of the top surface of the
substrate. The cover layer can contain a glass having a specific
caloric conductivity of >2 W/mK. The cover layer can have a
window disposed above a section of the fuse element, the section of
the fuse element located within the window being at least partially
covered by a solder containing layer.
Inventors: |
Jollenbeck, Andre; (Witten,
DE) ; Roder, Uwe; (Witten, DE) ; Baus,
Andreas; (Dortmund, DE) |
Correspondence
Address: |
Jay F Moldovanyi
Fay Sharpe Fagan Minnich & McKee
Seventh Floor
1100 Superior Avenue
Cleveland
OH
44114-2518
US
|
Family ID: |
7687801 |
Appl. No.: |
10/479880 |
Filed: |
April 16, 2004 |
PCT Filed: |
June 11, 2002 |
PCT NO: |
PCT/EP02/06392 |
Current U.S.
Class: |
337/227 ;
337/231 |
Current CPC
Class: |
H01H 85/046 20130101;
H01H 85/0056 20130101 |
Class at
Publication: |
337/227 ;
337/231 |
International
Class: |
H01H 085/143; H01H
085/165 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 11, 2001 |
DE |
101 28 108.0 |
Claims
1. A fuse device including: an electrically insulating substrate
with an upper surface; a thick film fusible conductor applied to
the upper surface of the substrate; and a cover layer comprising an
electrically insulating material of good thermal conductivity
applied directly to the thick film fusible conductor and adjoining
regions of the upper surface of the substrate.
2. A fuse device as claimed in claim 1, characterised in that the
specific thermal conductivity of the material of the cover layer is
greater than 2 W/mK.
3. A fuse device as claimed in claim 2, characterised in that the
cover layer is produced from a paste applied in a screen printing
process by tempering, the paste containing particles of at least
one substance from a good thermally conductive group of substances
including glasses, aluminium oxide, aluminium nitride and silicon
nitride.
4. A fuse device as claimed in claim 1, characterised in that the
cover layer is a sintered thick layer containing a glass.
5. A fuse device as claimed in claim 4, characterised in that the
sintered thick layer is produced from a glass frit by tempering at
a temperature between 750.degree. C. and 950.degree. C., preferably
about 850.degree. C.
6. A fuse device as claimed in claim 5, characterised in that the
cover layer is 10 .mu.m-100 .mu.m, preferably 20 .mu.m-40 .mu.m,
thick.
7. A fuse device as claimed in one of claims 1-6, characterised in
that the substrate is a ceramic substrate with good thermal
conductivity.
8. A fuse device as claimed in claim 7, characterised in that the
substrate is a ceramic Al.sub.2O.sub.3 substrate.
9. A fuse device as claimed in one of claims 1-8, characterised in
that the substrate has an elongate, substantially rectangular upper
surface, the thick film fusible conductor extending between two
connecting surfaces disposed at the narrow sides of the upper
surface, the connecting surfaces not being covered by the cover
layer.
10. A fuse device as claimed in claim 9, characterised in that the
upper surface has a width of between 1 mm and 4 mm and a length of
between 6 mm and 15 mm.
11. A fuse device as claimed in claim 9 or 10, characterised in
that the thick film fusible conductor has a width between the
connecting surfaces of between 0.1 mm and 1.5 mm.
12. A fuse device as claimed in one of claims 9-11, characterised
in that the thick film fusible conductor extends, at least in a
central section, in a serpentine shape between the connecting
surfaces on the upper surface of the substrate.
13. A fuse device as claimed in one of claims 9-12, characterised
in that the cover layer has at least one window, which is arranged
above a section of the fusible conductor, and that the section of
the fusible conductor situated in the window is at least partially
covered by a layer which contains a substance which, when heated,
can act on the fusible conductor situated beneath it such that the
electrical resistance of the section of the fusible conductor
increases.
14. A fuse device as claimed in claim 13, characterised in that the
substance is a metal, which can diffuse into the fusible conductor
and the layer containing the metal has a good thermal
conductivity.
15. A fuse device as claimed in claim 14, characterised in that the
fusible conductor contains silver and the substance includes lead
and/or tin.
16. A fuse device as claimed in one of claims 13-15, characterised
in that the entire section of the fusible conductor situated within
the window is covered by the layer.
17. A fuse device as claimed in one of claims 13-15, characterised
in that the thick film fusible conductor extends on the upper
surface of the substrate, at least in a central section between the
connecting surfaces, in a serpentine shape with alternating
straight and arcuate sections, that the window of the cover layer
is situated above an arcuate section and portions of the two
adjacent straight sections of the loop of the fusible conductor and
that at least the arcuate section of the fusible conductor is
covered by the layer containing the substance.
18. A fuse device as claimed in one of claims 1-17, characterised
in that a protective plastic layer is applied above the cover
layer.
19. A method of manufacturing a fuse device, wherein a thick film
fusible conductor is applied to an upper surface of an electrically
insulating substrate and a cover layer of an electrically
insulating material of good thermal conductivity is applied
directly onto the thick film fusible conductor and adjoining
regions of the upper surface of the substrate.
20. A method of manufacturing a fuse device as claimed in claim 19,
characterised in that, for the purpose of applying the thick film
fusible conductor, a paste is imprinted in a screen printing
process and the layer thus formed is tempered and that these
application steps are repeated at least once in order to increase
the thickness of the layer.
21. A method of manufacturing a fuse device as claimed in claim 19
or 20, characterised in that, for the purpose of applying the cover
layer, a paste is imprinted in a screen printing process and the
layer thus formed is subsequently tempered.
22. A method of manufacturing a fuse device as claimed in claim 21,
characterised in that the paste is a glass frit, which is tempered
after imprinting at a temperature of between 700.degree. C. and
900.degree. C., preferably about 850.degree. C.
23. A method of manufacturing a fuse device as claimed in claim 21
or 22, characterised in that the cover layer is so imprinted that
at least one window is formed in the cover layer above a section of
the fusible conductor, and that a layer is applied in the window,
at least above a portion of the section of the fusible conductor,
which contains a substance, which, when heated, can act on the
fusible conductor situated beneath it such that the resistance of
the section of the fusible conductor increases.
24. A method of manufacturing a fuse device as claimed in claim 23,
characterised in that the entire section, which is situated in the
window, of the fusible conductor is covered by the layer.
25. A method of manufacturing a fuse device as claimed in claim 23,
characterised in that the thick film fusible conductor is applied
to the upper surface of the substrate, at least in part, with a
serpentine shape with alternating straight and arcuate sections,
that the window is formed in the cover layer above an arcuate
section and portions of the two adjacent straight sections of the
loop of the fusible conductor, and that at least the arcuate
section of the fusible conductor is covered by the layer containing
the substance.
26. A method of manufacturing a fuse device as claimed in claim 24
or 25, characterised in that a layer containing a solder is
imprinted in the window and is then briefly melted.
27. A method of manufacturing a fuse device as claimed in claim 26,
characterised in that a solder layer of a thickness of between 70
.mu.m and 130 .mu.m is imprinted with the aid of a template.
Description
[0001] The invention relates to a fuse device, in which a thick
film fusible conductor is applied to an upper surface of an
electrically insulating substrate, and to a method of manufacturing
such a fuse device.
[0002] Fuse devices of the type referred to above are disclosed in
the prior art in a series of publications. Reference is made by way
of example to the fuse for SMD installation described in WO
96/41359 A1. Formed on a rectangular surface of an insulating
substrate, which consists, for instance, of Al.sub.2O.sub.3,
between two connecting surfaces, is a metallic thick film fusible
conductor. The connecting surfaces are formed on opposing edges of
the surface of the substrate and are composed of a plurality of
metal layers and are provided for the purpose of SMD installation
with a solderable coating. A spot comprising a layer, which
contains tin/lead, is applied to a central section of the fusible
conductor applied to the surface of the substrate. The
configuration is so designed that in the event of predetermined
current flows of predetermined minimum durations the fusible
conductor and the spot applied on it heat up to an extent which is
sufficient to soften or to melt the material of the spot to the
extent that the tin/lead metal diffuses into the metal of the
fusible conductor disposed beneath it. This locally increases its
electrical resistance, which results in an increased voltage drop,
an increased local power loss, further heating and finally in
melting and/or vaporisation of the material of the fusible
conductor. The current which results in the described manner in
rupturing of the fusible conductor is less than the current which
would be necessary for melting the fusible conductor without the
applied tin/lead spot. However, as a result of the described,
time-consuming processes, a considerably longer time of the current
flow is necessary until rupture (tripping); the fuse device is very
"sluggish".
[0003] On the other hand, U.S. Pat. No. 5,166,656 discloses a very
rapidly acting SMD fuse for protecting electronic circuits, in
which a metallic thin film fusible conductor with a thickness of
0.6 to 4.5 .mu.m is applied to a glass substrate and is covered
with a passivation layer of CVD SiO.sub.2 or imprinted glass,
whereafter a second glass plate is secured to it with an adhesive
layer (epoxide).
[0004] Slow acting fuses of small size are required, for instance,
in telecommunication devices, particularly to protect input
circuits or interface circuits, which are coupled to long
transmission lines. These transmission lines are subjected to the
influences of electric and magnetic fields which are produced by
lightening strikes and high voltage cables extending in the
vicinity. These influences can result, amongst other things, in
brief current/voltage pulses with high peak values on the
telecommunication signal transmission lines, which can potentially
damage the devices connected to them, particularly their input
circuits. The input connections of the device are thus protected
against over-voltages and, with the aid of fusible protection
devices, against excessive currents. These telecommunication
devices or their fuse devices are subjected to complicated
requirements, which are specified in a series of special tests. On
the one hand, "telecommunications" fuse devices should reliably
trip (that is to say no longer enable the flow of current even by
way of an arc) at currents of predetermined magnitude within
predetermined maximum current flow periods (e.g. at 40 A within 1.5
s or at 7 A within 5 s). Furthermore, the fuse devices should be
slow acting, that is to say if their maximum permissible current is
slightly exceeded they trip (rupture) after a relatively long
duration of the current flow. Finally, they should be able to
resist brief (in the millisecond range) relatively large currents
of up to 100 A without tripping (such currents are produced e.g. in
the event of over-voltage pulses, which are dissipated to earth by
an over-voltage protective device with a low internal resistance,
whereby the current which is produced flows via the fuse element).
The requirements on devices with "telecommunications" fuse devices
are specified e.g. in the "UL 1950", "FCC Part 68" and "Bellcore
1089" tests.
[0005] It is the object of the invention to provide a fuse device
which renders it possible to satisfy the requirements referred to
above with a small structural size and low manufacturing costs and
which furthermore can be constructed in the form of an SMD
component.
[0006] This object is solved by a fuse device with the features of
claim 1 and by a method of manufacturing a fuse device with the
features of claim 19.
[0007] The fuse device in accordance with the invention has an
electrically insulating substrate with an upper surface, a thick
film fusible conductor applied to the surface of the substrate and
a cover layer of an electrically insulating material of good
thermal conductivity applied directly to the thick film fusible
conductor and adjoining regions of the surface of the substrate. It
is possible with this arrangement to improve the resistance of the
fuse device to very briefly flowing high currents in a manner which
is simple to manufacture (namely a simple structure with few
layers). The cover layer has a number of complementary effects: it
stabilises the surface of the fusible conductor, it acts as a brief
thermal buffer (or thermal drain and store) and it can inhibit the
production and maintenance of an arc during and after tripping.
[0008] Electrically insulators generally have, in comparison to
conductive materials (such as metals), a poor thermal conductivity.
The term "good thermal conductivity" in the context of the
invention should therefore be understood as thermal conductivity
which is above average for an electrical insulator. The specific
thermal conductivity of the material of the cover layer should be
greater than 2 W/mK, preferably greater than 4 W/mK. The cover
layer is produced e.g. from a paste applied in a screen printing
process by tempering, the paste containing particles of at least
one substance from a good thermally conducting group of substances
including glasses, aluminium oxide, aluminium nitride and silicon
nitride. In another preferred exemplary embodiment, the cover layer
is a sintered thick film containing a glass which was produced from
a glass frit by tempering at a temperature between 700.degree. C.
and 900.degree. C., preferably about 850.degree. C. The cover layer
is preferably relatively thick, for instance 10 .mu.m-100 .mu.m,
preferably 20 .mu.m-40 .mu.m, thick.
[0009] The substrate is preferably a ceramic substrate with a good
thermal conductivity, for instance a ceramic Al.sub.2O.sub.3
substrate.
[0010] In a preferred embodiment, the substrate has an elongate,
substantially rectangular upper surface, the thick film fusible
conductor extending between two connecting surfaces disposed at the
narrow sides of the surface, the connecting surfaces not being
covered by the cover layer. The surface has e.g. a width between 1
mm and 4 mm and a length between 6 mm and 15 mm.
[0011] The thick film fusible conductor preferably has a width
between the connecting surfaces of between 0.1 mm and 1.5 mm.
[0012] This small substrate size for thick film fuse devices
permits a relatively large width (preferably in conjunction with a
relatively large layer thickness), a relatively large
cross-sectional area of the fusible conductor and thus a high
current capacity, which (and also the cover layer in accordance
with the invention) inhibits rupturing under brief current pulses
of high amplitude.
[0013] In a preferred embodiment of the fuse device, the thick film
fusible conductor extends, at least in a central section, between
the connecting surfaces in a serpentine shape (i.e. in loops in
opposite directions). It is thus possible to increase the length of
the fusible conductor, which has a relatively large cross-sectional
area, with a small size of the substrate surface. With this sizing
possibility, different rated currents can be achieved with
approximately the same momentary pulse resistance.
[0014] In a preferred embodiment of the fuse element in accordance
with the invention, the cover layer has at least one window, which
is arranged over a section of the fusible conductor. The section of
the fusible conductor situated in the window is at least partially
covered by a layer, which contains a substance, which, when heated,
can act on the fusible conductor situated beneath it such that the
electrical resistance of the section of the fusible conductor
increases. The window can be of any desired shape but, when
producing the layers by a screen printing process, is preferably of
approximately rectangular shape with edges aligned in the screen
printing direction. The window can be formed exclusively on the
fusible conductor layer or can be so wide that regions of the
substrate surface adjacent to the fusible conductor are also
exposed. The substance in the layer applied in the window is, for
instance, a metal, which can diffuse into the fusible conductor.
For instance, the fusible conductor contains silver and the
substance contains lead and/or tin. The arrangement is so designed
that in the event of predetermined current flows of predetermined
minimum durations, heating of the fusible conductor and the layers
applied thereon occurs, which is sufficient to permit the substance
in the layer to act on the fusible conductor disposed beneath it.
This locally increases its electrical resistance, which results in
an increased voltage drop, an increased local power loss, further
heating and finally in melting and/or vaporisation of the material
of the fusible conductor. The current intensity, which results in
the described manner in rupturing of the fusible conductor, is
smaller than the current intensity, which would be necessary to
melt the fusible conductor without the layer applied in the window.
However, as a result of the aforementioned, time-consuming
processes, a considerable longer time of the current flow is
necessary until rupturing (tripping) occurs; the fuse device
becomes more slow acting.
[0015] The layer containing the metal preferably has a good thermal
conductivity. This provides the possibility of rapidly dissipating
heat which is produced in the fusible conductor beneath it as a
result of momentary current pulses. The layer thus adopts a
function of the cover layer lacking in the window. The entire
section of the fusible conductor situated in the window is
preferably covered by the layer so that the entire fusible
conductor is covered either by the heat-dissipating cover layer or
by the layer applied in the window. The layer can furthermore
overlap with the edge of the window in order to compensate for
technologically determined tolerances.
[0016] In one exemplary embodiment, the thick film fusible
conductor extends, at least in a central section, between the
connecting surfaces in a serpentine shape with alternating straight
and arcuate sections on the surface of the substrate. The window in
the cover layer is disposed above an arcuate section and portions
of the two adjacent straight sections of the loop of the fusible
conductor and at least the arcuate section of the fusible conductor
is covered by the layer containing the substance. In this exemplary
embodiment, of the sections of the serpentine fusible conductor
exposed in the window (not covered by the cover layer), at least
the sections with the locally highest current densities (namely the
arcs) are covered by the layer (e.g. a solder layer) applied in the
window.
[0017] A preferred embodiment of the fuse device is characterised
in that a protective plastic layer is applied above the cover
layer. This consists preferably of a self-quenching plastic
material, e.g. a self-quenching epoxide resin.
[0018] In the method in accordance with the invention for
manufacturing a fuse device, a thick film fusible conductor is
applied to an upper surface of an electrically insulating
substrate. A cover layer of an electrically insulating material of
good thermal conductivity is applied directly to the thick film
fusible conductor and adjoining regions of the surface of the
substrate.
[0019] In order to apply the thick film fusible conductor, a paste
is preferably imprinted in a screen printing process. The layer
thus formed is tempered. These application steps are preferably
repeated at least once in order to increase the layer thickness.
The production of a relatively thick fusible conductor is thus
rendered possible, which permits a high current capacity, which
results in an improved pulse resistance (see the explanation
above). In order to apply the cover layer, a paste is preferably
also imprinted in a screen printing process and the layer thus
formed is subsequently tempered (fired). The paste is preferably a
glass frit, which is tempered, after imprinting, at a temperature
of between 700.degree. C. and 950.degree. C., preferably about
850.degree. C.
[0020] In a preferred embodiment, the cover layer is so imprinted
that at least one window is formed in the cover layer above a
section of the fusible conductor. A layer is applied in the window,
at least above a portion of the section of the fusible conductor,
which contains a substance, which, when heated, can act on the
fusible conductor disposed beneath it such that the resistance of
the section of the fusible conductor increases. In a preferred
embodiment, a solder-containing layer is imprinted in the window
and then briefly melted. A solder layer with a thickness of between
70 .mu.m and 130 .mu.m is preferably imprinted with the aid of a
template. This relatively thick solder layer creates a good local
thermal absorption buffer and an excess of the metals diffusing
into the fusible conductor.
[0021] Advantageous and preferred embodiments of the invention are
characterised in the dependent claims.
[0022] The invention will be described below in more detail with
reference to preferred embodiments illustrated in the drawings, in
which:
[0023] FIG. 1 is a schematic plan view of a first embodiment of a
fuse device in accordance with the invention with cover layers
partly cut away;
[0024] FIG. 1a is a sectional view of the fuse device of FIG. 1
along the line A-A;
[0025] FIG. 1b is a sectional view of the fuse device of FIG. 1
along the line B-B;
[0026] FIGS. 2a-2d are schematic views of a substrate with layers
applied thereon, which illustrate method steps in the manufacture
of the fuse device shown in FIG. 1; and
[0027] FIGS. 3a-3d are schematic views of a substrate with layers
applied thereon, which illustrate method steps in the manufacture
of an alternative embodiment of the fuse device in accordance with
the invention.
[0028] FIG. 1 is a schematic plan view of a fuse device 10 in
accordance with the invention, the upper layers being partially cut
away for reasons of visualisation. FIGS. 1a and 1b are sectional
views of the fuse device 10 shown in FIG. 1, the section being on
the line A-A and B-B, respectively. The fuse device 10 is produced
on a substrate 12. In the preferred embodiment, the substrate
comprises an Al.sub.2O.sub.3 ceramic with a thickness between 0.5
mm and 0.7 mm, for instance 0.63 mm. The substrate 12 illustrated
in FIG. 1 of the preferred exemplary embodiment is about 10 mm long
and 2.5 mm wide. The illustrated substrate chip is preferably cut
out from a larger substrate wafer, whereby a plurality of fuse
device chips arranged in rows and columns can be fabricated
simultaneously on the substrate wafer.
[0029] Applied to the upper surface of the substrate 12 shown in
FIG. 1 is a thick film fusible conductor 14. The fusible conductor
14 comprises a layer, which is applied by screen printing and
sintered, of adjoining silver particles and preferably has a
thickness of about 20 .mu.m. Such a thickness is produced, for
instance, by successively imprinting two layers of 10 .mu.m
thickness each, whereby after imprinting the first layer it is
firstly fired before the second layer is imprinted. The thick film
fusible conductor 14 has a serpentine shape, the width of the
fusible conductor in the serpentine region being about 0.2 mm. In
the vicinity of the narrow sides of the substrate 12, the fusible
conductor 14 adjoins contact surfaces 16. The contact surfaces 16
can also be produced from the film of the fusible conductor 14
and/or from further films. The contact surfaces 16 extend around
the outer edges of the substrate with the exception of the (not
shown in FIG. 1) underside of the substrate 12. The contact
surfaces 16 preferably comprise a galvanically produced layer
system with a subsequently applied solder layer.
[0030] Applied above the fusible conductor 14 and the adjacent
exposed regions of the upper surface of the substrate 12 is a cover
layer 18. In the exemplary embodiment shown in FIG. 1, the cover
layer 18 covers nearly the entire surface of the substrate 12 with
the exception of the contact surfaces 16 and a window 20 (which
will be described below in more detail). The cover layer 18 is
preferably produced with the aid of a screen printing process, in
which a glass frit is imprinted and subsequently tempered (fired)
so that a thickness of the cover layer of e.g. about 20 .mu.m is
produced. The components of the glass frit are so selected that a
layer with a relatively good thermal conductivity forms. In the
exemplary embodiment illustrated in FIG. 1, the cover layer does
not extend to the longitudinal sides of the substrate 12 so that,
on a substrate wafer with a plurality of chips arranged in rows and
columns, strips remain between the chips which are free of the
cover layer 18. These strips can serve to optically mark the chip
borders and facilitate separation. Furthermore, spacing of the
cover layer from the parting region between the chips prevents a
negative influence on the layer 18 by the separation process (e.g.
sawing or scoring/cracking).
[0031] As already mentioned, the cover layer 18 has a window 20.
The window 20 is so arranged that a loop of the serpentine fusible
conductor is exposed in the window, the loop comprising a curve and
straight sections connected to it of the fusible conductor. The
window 20 is preferably arranged approximately in the middle of the
fuse device 10. With an approximately symmetrical construction of
the serpentine fusible conductor 14, the region of the strongest
heating is produced in the centre of the fuse device 10. A layer 22
is applied in the window 20 above the curved portion of the section
of the fusible conductor exposed in the window, the layer 22 being
produced by imprinting a solder-containing paste with the aid of a
printing template and subsequent heating until the solder
components briefly melt. The solder-containing layer imprinted in
the template has, for instance, a thickness of about 100 .mu.m.
After the brief melting, a permanent drop-shaped construction is
produced after the cooling process as a result of the surface
tension of the molten material, which is shown, for instance, in
FIG. 1a. The solder material contained in the layer 22 is, for
instance, a tin/lead alloy. In addition to tin and lead, further
metals can be included in the alloy. In the exemplary embodiment
shown in FIG. 1, the window 20 extends 1 mm in the longitudinal
direction of the substrate 12 and is about 1.5 mm wide. The layer
applied in the window is about 0.7 mm wide and extends
substantially over the entire length of the window.
[0032] The entire structure comprising the fusible conductor 14,
cover layer 18 and the layer 22 applied in the window 20 is covered
by a protective layer 24. However, the protective layer 24 leaves
the contact surfaces 16 exposed. The protective layer 24 preferably
consists of an epoxide reside, preferably a self-quenching epoxide
resin. With the substrates referred to above, the thicknesses
referred to above of the layers applied thereon and a thickness of
the protective layer of less than 1 mm, the total thickness of the
fuse device 10 thus produced remains significantly below 2 mm, so
that the device satisfies the requirements of the mini-PCI shape
factor.
[0033] The serpentine thick film fusible conductor 14 shown in FIG.
1 has a relatively large width and a relatively high thickness in
order to provide an adequate current capacity for an improved pulse
resistance. The serpentine shape permits a relatively large length
of the fusible conductor resistance on the substrate 12 to be
produced. Fuse elements 10 with different rated currents can be
designed by differing resistance lengths. In a preferred
embodiment, the fuse element has, for a rated current of 1.5 A, for
instance, a resistance of about 90 m.OMEGA. and, for a rated
current of 2 A, a resistance of 60 m.OMEGA..
[0034] Different views of the substrate 12 of the fuse element 10
with layers applied thereon are shown in FIGS. 2a to 2d in order to
show the sequence of the application of the individual layers in
the manufacture of the fuse element.
[0035] Reference is made in each case to a chip in FIGS. 2a to 2b
in the following description of the manufacture of the fuse device
described with reference to FIG. 1. Reference is also made at this
point to the fact that the described method steps are preferably
performed on a substrate wafer which has a plurality of chips of
the illustrated type arranged in rows and columns. The layers are
thus applied simultaneously for a plurality of chips.
[0036] The layer 14 containing silver is firstly applied to the
upper surface of the substrate 12 in a screen printing process. At
its two ends, the fusible conductor layer 14 has diverging areas 26
which adjoin the contact surfaces 16. In the exemplary embodiment
illustrated in FIG. 2a, all the bends are of the same length with
the exception of the serpentine loop arranged in the centre. The
serpentine loop arranged in the centre, which is subsequently to be
covered with a solder layer, is displaced somewhat from the edge of
the substrate 12 in order to achieve a better position of the
solder spot and of the protective layer, as will be described in
more detail below. After imprinting the layer 14, it is fired. A
second fusible conductor layer is then imprinted with the same
layout onto the fired first layer in order to achieve a greater
thickness of the fusible conductor and is again fired.
[0037] The cover layer 18 is then imprinted onto the fusible
conductor layer 14, imprinted on the substrate 12 and fired, as is
shown in FIG. 2b. In the present preferred exemplary embodiment a
glass frit is applied in a screen printing process and subsequently
tempered (fired) at a temperature of about 850.degree. Celsius in
order to produce a layer with a thickness of about 20 .mu.m. The
glass frit which is used with a firing temperature of 850.degree.
Celsius is commonly referred to as a "high-melting glass layer",
since the firing or sintering temperature of 850.degree. Celsius is
above the firing temperature of about 500.degree.-600.degree.
Celsius used with the glass frits which are otherwise common. The
glass layer formed therefrom has a relatively high (for electric
insulators) specific thermal conductivity of more than 3.5 W/mK,
for instance a specific thermal conductivity of 4.3 W/mK. The cover
layer 18 has a window 20, which, in this embodiment, is arranged at
the edge of the cover layer 18 and is thus defined by only three
edges. The central, shortened serpentine loop illustrated in FIG.
2a is arranged in the window 20.
[0038] After the tempering of the cover layer 18, a
solder-containing layer 22 is imprinted by means of a template in
the window 20 above the serpentine loop disposed therein. The layer
22 produced by stencil printing preferably has a thickness of about
10 .mu.m. The layer 22 is so disposed within the window 20 that it
completely covers the arc of the serpentine loop, whereby remaining
between the edge of the solder-containing layer 22 and the edge,
extending in the longitudinal direction of the window 20 there is a
space, in which the two straight sections of the fusible conductive
layer 14, which are connected to the arc of the bend, are exposed,
i.e. are covered neither by the cover layer 18 nor by the
solder-containing layer 22. This results in the exposed sections of
the fusible conductive layer 14 being able to be subjected to a
higher thermal loading since a thermally dissipating cover is
missing in this region. This may, however, be less critical with
the straight sections of the serpentine fusible conductor 14
because the highest (because they are asymmetrically distributed)
current densities are produced in the arcuate sections.
[0039] In a following method step, the layer structure thus
produced is covered with a protective layer 24, for instance with
an epoxide resin layer. In this exemplary embodiment, the
protective layer has a thickness of up to 0.5 mm. After separation
into strips of chips connected together at their longitudinal
sides, the edge regions, including the connecting surfaces 16, of
the fuse devices, including the edges, are galvanically metallised.
A solder layer is applied to the galvanically applied sequence of
layers in order to ensure good solderability of the devices thus
produced. FIG. 2d shows the underside of the fuse device 10 thus
produced. The contact surfaces 16 engage the underside of the
substrate 12 around the sides and constitute their surfaces
suitable for soldering.
[0040] Schematic views of a substrate with layers applied thereon
are shown in FIGS. 3a to 3d, which illustrate method steps in the
manufacture of an alternative embodiment of the fuse device in
accordance with the invention. Since the method steps, i.e. the
sequence of the application of the layers, do not differ from those
described with reference to FIGS. 2a to 2d, only the differences
will now be described.
[0041] Firstly, the layout of the fusible conductor layer 14 shown
in FIG. 3a differs from that of the fusible conductor layer 14
shown in FIG. 2a. In the embodiment shown in FIG. 3a, all the
serpentine loops are of the same length.
[0042] In the embodiment of FIG. 2a, the contact surfaces 16 are
constituted by a separate metal layer, which is connected to the
layer of the fusible conductor 14. In the embodiment of FIG. 3a,
the contact surfaces 16 and the fusible conductor are constituted
by the same layer.
[0043] The window 20 in the cover layer 18 has a smaller width of
only about 0.7 mm in the embodiment of FIG. 3b so that
substantially only the arcuate section of the central serpentine
loop is exposed in the window. Furthermore, in the embodiment of
FIG. 3b, the solder-containing layer 22 is so applied that it
extends to at least the edge of the window 20 parallel to the
longitudinal sides so that the entire fusible conductor is covered
either by the cover layer or by the solder-containing layer 22.
This is currently the preferred embodiment; it ensures that all the
sections of the thick film fusible conductor 14 are covered by a
layer which dissipates heat.
[0044] Since the central serpentine loop disposed in the window is
not shortened and the solder-containing layer 22 is thus situated
relatively distant on the edge of the fuse device, the protective
layer 24 is displaced outwardly somewhat in the central region in
order reliably to cover the solder spot 22, as is shown in FIG. 3c.
Alternatively, the cover layer 14 can be moved as a whole further
towards the edges of the substrate 12.
[0045] The invention has been described above with reference to the
currently preferred embodiments. Numerous alternative embodiments
are, however, possible within the scope of the inventive concept,
as will be apparent from the attached claims.
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